Transformer apparatus



April 19, 1965 L.. A. MEDLAR 3,247,450

TRANSFORMER APPARATUS Original Filed March l5, 1957 3 Sheets-Sheet 1SZyl] MS/C f fok/Miky w 7%55.

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BY M l ATTORNEYS April 19, 1966y L. A. MEDLAR 3,247,450

TRANSFORMER APPARATUS BYMQZZM ATTORNEYS April 19, 1966 MMEDLAR 3,247,450

TRANSFORMER APPARATUS Original Filed March l5, 1957 3 Sheets-Sheet 5INVENTOR BY MQ@ ATTORNEYS United States Patent O 3,247,450 TRANSFORMERAPPARATUS Lewis A. Medlar, Lansdale, Pa., assignor to Fox ProductsCompany, Philadelphia, Pa., a corporation of Pennsylvania Originalapplication Mar. 15, 1957, Ser. No. 646,429, new Patent No. 2,999,973,dated Sept. 12, 1961. Divided and this application Aug. 21, 1961, Ser..No. .132,4'f6

Claims. (Cl. 323-60) This invention relates to a transformer system forsupplying power from an A.C. input to a load, and, more particularly, toa transformer system for supplying a substantially constant current to aload subject to variations of impedance. This application is a divisionof my copending application Serial Number 646,429, tiled March l5, 1957,and now US. Patent 2,999,973.

As will be understood after the following explanation, the transformersystem of this invention is capable of supplying a large number ofdifferent characteristic outputs. Among such outputs are acharacteristic rise of current output with increasing load, acharacteristic drop of current output with increasing load, and asubstantially constant current output with increasing load. Theinvention will be more fully described in conjunction with thelast-mentioned characteristic, which is preferred, but it will beunderstood that the invention is not limited to this characteristic.

In the past, several different means of obtaining a substantiallyconstant current output with changing load impedance have been evolved.Perhaps the earliest of these Various systems is that using a movablecoil, the coil being counter-balanced and automatically adjusted inaccordance with changing load to vary the distance between the input andoutput sides of the transformer and thereby to vary the coupling. Thissystem is still in use today for constant current lighting systems, butit is subject to several disadvantages, among which are poor speed ofresponse, the substantial expense of the system, the bulkiness of theunit, and the use of moving parts subject to Wear and eventual shutdownof the system.

Another past-suggested constant current apparatus uses a saturable corereactor. The reactor may be a relatively simple one or may be a complexsystem utilizing shunts, partial air gaps, etc. This type of system,however, is relatively expensive, as well as being complex. Moreover, ithas a lagging power factor.

A third general type of known constant current system is one using aresonance phenomenon. This type of system has taken many forms, but thesimplest form is the series resonant circuit, including an inductivereactance and a capacitive reactance connected in series, the tworeactances being adjusted to be substantially equal, and the load beingconnected across one of the reactances. While this type of systemprovides quite good constancy of output current, it has been foundnecessary to provide auxiliary means to limit the voltages across thecomponents on open circuit. Moreover, this type of system is not readilyadjustable to change the level of output current which is to tbemaintained constant. ln order to provide for `such change, there must bea change in both the actual inductance and the actual capacity of thecomponents.

Another adaptation of the resonant system is the socalled monocyclicsquare. This system is subject to several of the disadvantagesenumerated above for the simple series resonant system.

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In contrast to all of the above previously suggested systems, thetransformer system of the present invention provides a substantiallyconstant current output at high efficiency, with no moving parts, withno need for auxiliary units to limit component voltages on open circuit,with relativ-ely easy adjustment of the parts to change the level ofoutput current to be maintained constant, and for a substantially lowercost than many of the previous systems.

The transformer system of this invention is in the nature of a resonantphenomenon, but, as will be obvious from the theoretical analysis tofollow, it is far from a simple resonant phenomenon. None of the variousembodiments of the invention to be described uses a simple seriesresonant circuit, and all of the embodiments actually automaticallyadjust themselves to maintain a substantially resonant condition evenwith change of only one of the components, such as change in capacity.The system is not dependent for its operation upon leakage reactance oron saturation of the core or any of its components, and the systemactually is limited and adversely aected in its operation by theseever-present efects.

One important advantage of the transformer system of this invention isthe fact that it draws a leading power factor current from the input,rather than the lagging power factor usually obtained with a transformersystem.

The apparatus of the present invention is capable of use wherever it isdesired to draw a substantially constant current through a variableload, when a constant voltage input is provided. One instance of suchuse is in lighting systems.

The apparatus of the present invention, generally described, includes aninput, an output, and a control electrical circuit, each of thesecircuits including at least one coil which may be wound on a transformercore. The coils of the input and output electrical circuits are soinductively related, wound and connected with respect to one anotherthat two magnetic circuits, an input and an output, are formed. Themagnetic circuits have two common portions, in one of which themagnetomotive forces generated by the input and output currents aid, andin the other of which the magnetomotive forces generated by thesecurrents oppose. As a result, there is no direct coupling of powerbetween the input circuit and the output circuit. The control electricalcircuit includes a capacitor and is inductively coupled to one of thecoils of the input and output circuits through one of the commonportions of the magnetic circuits. Current through the control circuitgenerates a magnetomotive force which is opposite in phase and ofgreater magnitude than the output magnetomotive force in the commonportion through which the coupling takes place. The controlmagnetomotive force in effect reverses the output magnetomotive force insaid common portion of the magnetic circ-uit and hence couples powerfrom the input to the output through the control electrical circuit.

The invention will now be described in conjunction with the accompanyingdrawings, showing preferred embodiments thereof.

ln the drawings:

FIG. l is a schematic diagram of a transformer apparatus constructed inaccordance with the invention;

FIG. 2 is an equivalent schematic circuit diagram of the apparatus ofFIG. l;

FIG. 3 is a vector diagram to be used in explaining the operation of theapparatus of the subject invention;

FIG. 4 is a schematic diagram of an apparatus simiiar to that of FIG. lbut with the positions of the input and load reversed;

FIG. 5 is a schematic diagram illustrating the use of a control coil andthe vario-us possible connections thereof;

FIG. 6 is a schematic diagram of the apparatus of FIG. l with additionalmeans for varying the effective capacity in the control circuit;

FIG. 7 is a schematic diagram of a transformer apparatus similar to thatof FIG. 1 but embodying transformer coupling of the capacitor in thecontrol circuit;

FIG. 8 is a schematic diagram of a transformer apparatus similar to thatof FIG. 1 but embodying a saturable transformer to vary the effectivecapacitance in the control circuit; and

FIG. 9 is a graphical representation of the various types ofcharacteristics that can be obtained with transformer apparatusconstructed in accordance with the invention.

desired, however, the voltage input may or may not be constant,depending upon the conditions and the characteristics to be obtained.Moreover, the impedance Z may be of any type. However, if a constantcurrent output is desired, the load should be mainly resistive, and, forbest constancy, loads of substantially unity power factor are preferred.

The equivalent electrical circuit of the apparatus of FIG. l is shown inFIG. 2, in which the voltage input is represented as Ep, the primarycurrent by Ip, the output or load ycurrent by Io and the controllcurrent as IL. The mutual inductance or inductive coupling betweencoils L1 and L2 is represented by M2, between coils L2 and L3 by M1, andbetween coils L1 and L3 is represented by M3.

Using p,=jw, setting up mesh equations for the input, output and controlcircuits, assuming one2one turns ratios, neglecting ohmic resistance ofcoils, leakage reactance and core loss, and solving for the loadcurrent, we obtain the following equation:

Referring now to the drawings in detail, FIGS. 1-8 illustrateembodiments of the invention each including a three-legged shell-typecore having a separator of nonmagnetic material splitting the center leginto two parallel sections, with the result that the two outer legs areseparated by a non-magnetic gap. One coil is wound about the twoseparated center-leg sections, while the two other coils of the basicembodiments are wound around the two outer legs. The core is preferablyof ferromagnetic material, and stamped laminations of standardtransformer iron may malte up the core.

It is not absolutely necessary that ferromagnetic material oe used forthe core, if the increased reluctance and leakage of non-ferromagneticmaterial is not too important or is compensated for by the properselection of the other parameters of the systems. However, aferromagnetic corc, because of its high permeability, but not because ofits saturation characteristics, is preferred.

Referring iirst to FIG. l, that gure discloses a transformer systemincluding a shell-type core 70 of ferromagmetio-material having centerleg 'll divided into two portions 71a and 71h by a strip of non-magneticmaterial 72. Any appropriate non-magnetic material may be used, or theseparator may be an air gap. A coil L1 is wound around both of parallelsections 71a and 71b of center leg 71. Outer legs 73 and 74 have coilsL2 and L3 wound therearound.

It is not necessary that the legs of the core be coplanar, or parallel,as shown in the drawings. Moreover, it is not necessary that thenon-magnetic gap formed by separator 72 be the only gap in the core. Itis only necessary that magnetic paths be formed between the two outerlegs and the adjacent portions of the center leg.

Coils L2 and L3 are connected in series by conductor 75, and the seriescombination of these coils is connected to a load 76 of impedance Z byleads 77 and 78. Coils L2 and L3 are so wound on the outer legs andconnected with respect to the direction of primary flux that theirvoltages oppose in the output circuit.

Coil L1 forms the input electrical circuit and is connected across asource of constant A.-C. voltage 79.

Coils L2 and L3 form the output electrical circuit, and the parallelcombination of coil L3 andcapacitor K forms the control electricalcircuit.

The apparatus of FIG. l is capable of providing several different typesof output characteristics, but it is preferred that it be used toprovide a substantially constant current output with variation in loadimpedance. For such use, the voltage of the input must be maintainedsubstantially constant. If constant current output is not It is evidentthat the above equation is relatively complicated and not susceptible ofeasy analysis, but it will be noted that the impedance value Z appearsin only one term, that being in the denominator of the equation. For theload current I0 to be independent of load, the multiplier of theimpedance Z must be zero, so we obtain:

L1 2 2 p KJVLlL-M,2

This obviously is the condition for pure constancy of the load, oroutput, current. Through simple arithmetical operations on the lastequation, we obtain:

@L3-M32 1 (3) wl L1 :ITK

This last equation is obviously for a resonant system, but it also isobviously not a simple resonance equation.

The apparatus of FIG. 1 forms two magnetic circuits, split into twoseparate closed loops by the non-magnetic separator, while the secondmagnetic circuit is identical with the lirst magnetic circuit. It willbe evident that the two magnetic circuits have two common portions, thetwo separated closed magnetic paths being these two comm-on portions.

The current flowing through the primary coil L1 generates amagnetomotive force Fp. There is also an output magnetomotive force Fowhich generates iluX which follows the same path as the primarymagnetomotive force. It will be noted that magnetomotive forces Fo andFp aid in the righthand common portion of the two magnetic circuits,while they oppose in the other lcommon portion. Consequently, there canbe no coupling of power directly between the input and the load.However, the control electrical circuit causes a control magnetomotiveforce FL to be generated in the righthand common portion of the twomagnetic circuits, which magnetomotive force is much greater than theoutput magnetomotive force in this leg and opposes it, with the resultthat the resultant of the control and output magnetomotive forces inthis leg is effectively reversed from the output magnetomotive force andcauses coupling of power between the input and the load through thecontrol electrical circuit.

In general, operation of the transformer apparatus. above described issimilar to that explained in detail inz my copending application SerialNumber 646,429, filed. March 15, 1957, now U.S. Patent 2,999,973, ofwhich the present application is a division, and said copendingapplication sets out in detail a vector analysis and characteristiccurves applicable to the apparatus here, described.

FIG. 3, which was shown and described as FIG., 6v of that copendingapplication, is designed to give a complete picture of the action of theideal transformer, showing all important component voltages and currentsthereof, for increasing impedance. It will be noted that 4as theimpedance increases continuously, the Vector net E3 will rotateclockwise, decreasing continuously. Since the control current IL isdriven by net E3 and the voltage across the control coil which is inphase with net E3) the control coil current must rotate and decreasecontinuously with net E3. Likewise, the control m.m.f. will decreasecontinuously with net E3 and, E0, being substantially 90 behind net E3,will rotate clockwise. The output current is directly dependent upon thecontrol current, so that, as the control current decreases, the outputcurrent likewise decreases continuously, describing a semi-circle ofdiameter equal to the original output current at zero impedance.

It will also be noted that the primary current Ip is substantially inphase with the control coil current IL, and so leads the primary voltageEp. This results in a leading power factor ordinarily an advantageouscondition.

It will be observed from the vector diagram of FIG. 3 that the idealtransformer tends toward a continuously decreasing output current withincreasing load impedance, rather than a constant output current.However, an ideal transformer has been stipulated, in which negligiblem.m.f. is required to drive the fluxes necessary to produce the voltagesin the system, that is, the core material has zero reluctance. Theinductances of the coils would be infinite but the mutual inductancebetween the coils, due to their physical separation, would be finite.Therefore the second term of Equation 2 would be zero so that it couldnever balance the first term. Hence the impedance would play a verylarge part in the operation of the system and the output current wouldvary inversely therewith.

In actual practice, however, the inductances of the coils are neveriniinite and it has been found that the coupling, as represented by M inEquation 2, automatically adjusts itself in correspondence with theValue of capacity to make the equation substantially true.

To bring out this compensating feature of practical transformers inwhich the coupling is not perfect and in which the core has appreciablereluctance, consider next a different ideal core material havingappreciably less than perfect coupling and constant reluctance, but notsubject to saturation effects. With such a core material, the net m.m.f.must be appreciable to drive the circulating tlux which links coils L3`and L3. There must be relative rotation between the control and outputm.m.fs toward Basic E3 to produce this appreciable net m.m.f. In otherwords, the control and output m.m.f.s are no longer substantially at 180to each other, and the net m.m.f. must therefore rotate back towardBasic E3 from the phase relationship in which it was substantially inphase with net E3.

For this new ideal core material, the relative rotation between thecontrol and output m.m.f.s required to produce the linearly increasingnet m.m.f. with increasing `output Voltage will tend to maintain theoutput current constant. If the reluctance is of poper value, the netrn.m.f. will stay midway between Basic E3 fand net E3 despite increasingoutput voltage and clockwise rotation of net E3. The output current thenwould be perfectly constant over a range of output voltage limited onlyby leakage between the windings. If the reluctance was too high, ofcourse, the required net m.m.f. would be so high that it would be closerto Basic E3 than to net E3 and the output current would rise withincreasing Voltage. If the reluctance was too low, the system wouldapproach the first ideal core having negligible reluctance, and`thecurrent would drop off. However, it is the apparent reluctance, asrepresented by the coupling, rather than the physical reluctance, thatis important. The value of this apparent reluctance relative to theother parameters of 6 the system, then, determines the characteristic ofoutput current versus output voltage.

The apparatus of FIG. 1 was connected so that the input was shunteddirectly across coil L1, while the output was connected across theseries combination of coils L3 and L3. Referring now to FIG. 4, it isshown therein lthat the positions or connections of the input and loadin the system may be reversed. In other words, input 79 may be connectedacross the series combination of coils L2 and L3, while load 76 may beconnected across L1. The operation of this form of the invention issubstantially the same as the operation of the apparatus of FIG. 1.

In FIG. 5, it is shown that a separate control coil may be used for thecontrol electrical circuit. This coil, L4, is wound on the same leg withcoil L3, and is connected in series with capacitor C. The leads 81 and82 of the control electrical circuit formed by the series combination ofcontrol coil L4 and capacitor C may be connected across any appropriatesource of voltage including coil L1, L3, L3, the series combination ofL3 and L3, any other source of voltage, or the two leads may be shortedtogether. For best range of constancy, however, the phase relationshipsshould be such that the control current is substantially 90 out of phasewith the primary voltage at zero load.

The increased number of turns of the control circuit obtained with aseparate control coil increases the voltage available to drive thecontrol current, thus increasing the magnitude of that current. Sincethe output current is directly related to the control current, anincrease in control turns increases the level of output current. The useof the separate control coil may be interpreted into the above equationsby substitution of the actual value of the capacity C in the equationfor the effective capacity K, using the equivalency K=2C, where 1-{orand a is the turns ratio between coils L3 and L4.

It has been found that output current regulation, in transformerapparatus constructed in accordance with the invention, is substantiallythe same for each preselected value of capacitance in the controlcircuit, in contrast to the usual resonant circuit. Also, the mutualinductance apparently changes automatically to maintain 4the properapparent reluctance of the system to provide constant current output.

For commercial applications, it is desirable that the level of outputcurrent to be maintained constant be selectable. In order to provide forsuch selection, the modiiication of FIG. 6 may be employed. Thismodification provides a variable inductor 83 shunted directly acrosscapacitor K. Variation of the inductance of this inductor will vary theeffective capacity in the control electrical circuit, thereby permittingvariation of the output current.

It is also possible to couple the capacitor into the control electricalcircuit indirectly, rather than by placing the capacitor directly inseries with the control coil. This may be done, as in FIG. 7, byconnecting a transformer T :into the control electrical circuit. Thesecondary of that transformer is connected directly across capacitor C,and the primary is connected across the control coil L3. Moreover, thecapacitor may be coupled 'into the control circuit through a variabletransformer, or Variac, to permit adjustment of effective C throughchange in transformer turns ratio.

FIG. 8 shows a further embodiment of the invention which provides fortransformer coupling of the capacitor into the control electricalcircuit, as well as variation of the effective capacity in the circuit.This embodiment uses a saturable transformer core 85 having a secondary,winding 86. Capacitor C is shunted across Winding 86.

Coils 87 and 83 are wound around the two outer legs of the satufrabletransformer core and are connected in series. The distal ends of thesecoils are connected to a source of variable D.-C. voltage 90, with theresult that variation in the voltage applied to coils S7 and 88 bysource 90 changes the level of saturation of core 85, and hence theprimary exciting current. Wound on the center leg with secondary winding86 is a primary winding 91 which is connected directly across controlcoil L3. The change in primary exciting current causes a change in theeffective capacity in the control electrical circuit, thus changing thelevel of output current which may be maintained constant by thisapparatus.

The invention has been described primarily for use in maintaining aconstant current output with variationV in -load impedance fromupwardly. However, the various embodiments described are useful forproviding other characteristics than constant current output. FIG. 9illustrates some of the characteristics of output current versus outputvoltage which can be obtained in accordance with ythe invention, withvariation of the control coil connections and of different parameters ofthe sys,- tem. It will be evident that a rising current characteristic,a drooping current characteristic, and various combinations of these twogeneral types of characteristics, as well as a constant current output,can be obtained. Accordingly, the invention is not to be consideredlimited to use of the apparat-us to obtain a constant current loadsince, in its broadest aspects, the invention is usable to obtain manyother different types of desirable characteristics.

As explained above, all of the embodiments of the invention have asubstantially unlimited constancy action with variation in capacity ofthe control circuit capacitor. When leakage reactanee is ignored, allembodiments described have unlimited constancy. This surprising resultis achieved by automatic variation in the mutual inductance of coils onseparate legs to compensate for changes in capacity, so that resonanceis substantially maintained. As far as is known, the action oftransformer systems in accordance with 4the invention in varying mutualinductance automatically, without adjustment of parts,is novel.

It will be obvious that many minor variations could be made in theelements of the various embodiments of this invention shown anddescribed without departure from the spirit of the invention. Forinstance, the positions of the various coils on the respective legs ofthe transformer core could be changed about and the characteristicsfundamental to the system still be maintained. From the description ofthe various embodiments, it will be evident that all of theseembodiments have the following in common:

The embodiments include two magnetic circuits, which have two commonportions, in one of which common portions the input and outputmagnetomotive forces aid, and in the other of these two common portionsthe input and output magnetomotive forces oppose, so that there is nodirect coupling between the input and the load. The controlrnagnetomotive force in effect reverses the total or resultantmagnetomotive force in its `common portion from the direction of theoutput m.m.f., so as to couple power from the input to the load throughthe vcontrol electrical circuit. From the above, it will be obvious thatthis invention is not to be considered limited to the variousembodiments shown and described, but rather only by the scope of theappended claims.

I claim:

1. A transformer system for supplying power to a load from a constantvoltage A.C. input comprising a core of a single type of ferromagneticmaterial having two outer legs and a center leg, said legs being of suchconfiguration and so aligned with each other that substantially closedhigh permeability paths are formed between the outer legs and the centerleg, said center leg being separated by non-magnetic material into twoparallel portions also parallel to said portions of the outer legs, sothat the reluctance of the path between said parallel portions of thecenter leg is extremely high in cornparison with the reluctance of saidsubstantially closed paths; at least three coils wound on said core, afirst and a second of said coi-ls being wholly wound on different onesof said outer legs and connected in series, the third coil being woundwith each turn surrounding both of and only said parallel portions ofthe center leg; an input, an output, and a control electrical circuit,all passing current, the input circuit including one of (a) said thirdcoil and (b) the series combination of said first and vsecond coils, theinput circuit being connected across said constant voltage A.C. input,the output circuit including the other one of (a) said third coil and(b) the series combination of said first and second coils, the outputcircuit being connected across said load; said input and outputelectrical circuits forming with said core an input and an outputmagnetic circuit, both including one of said parallel portions of thecenter leg and the outer leg with which a substantially closed highpermeability path is formed and the other of said parallel portions ofthe center leg and the other outer leg, said input and output electricalcircuits being so inductively related to said outer legs that the inputand output magnetomotive forces generated by the input and outputcurrents aid in one and oppose in the other of said outer legs; and acapacitive reactance, said control electrical circuit including saidcapacitive reactance and being inductively coupled to the leg on whichsaid first coil is wound; whereby no ypower is 4directly coupled betweensaid input and output electrical circuits, but the control currentproducing a magnetomotive force in the leg on which said first coil iswound of phase and magnitude to produce a resultant of the cont-rol andoutput magnetomotive forces opposite to the output magnetomotive forcein said last-mentioned leg and theretby to couple vpower from the inputto the load through said control electrical circuit.

2. The apparatus of claim 1 in which said control electrical circuitincludes said first coil and said capacitive reactance is shunted acrosssaid coil.

.3. The apparatus of claim l in which said control electrical circuitincludes a control coil wound on the leg on which said one of said rstand second coils is wound.

4. The apparatus of claim 3 in which the series combination of saidcontrol coil andsaid capacitive reactance is yconnected across a sourceof voltage.

S. The apparatus of claim 4 in which the series combination of saidcon-trol coil and said capacitive reactance is connected across saidfirst coil.

6. The apparatus of claim 3 in ywhich the series combination of saidcontrol coil and said capacitive reactance is connected across saidfirst coil.

7. The apparatus of claim 3 in which the capacitive reactance is shuntedacross thecontrol co-il.

8. The apparatus of claim l in which said control electrical circuitincludes a variable inductive reactance shunited across Ithe capacitivereactance to permit variation of the effective capacity and hence of theoutput current.

9. The apparatus of claim 1 in which said control electrical circuitincludes a saturable core transformer, at least one coil wound on thecore of said transformer, a source of D.C. voltage connected to saidlast-mentioned coil and variable to change the saturation of thetransformer core, a `secondary coil wound on said transformer core andconnected across said capacitive reactance, and a primary coilinductively coupled to said secondary coil.

1d. The apparatus of claim l in which said control electrical circuitincludes a transformer lhaving a primary and a secondary coil, saidcapacitive reactance being connected across said secondary coil, andsaid primary coil being inductively coupled to said secondary coil.

(References on following page) 9 10 References Cited by the Examiner2,605,457 7/ 1952 Peterson 323-6 X UNITED STATES PATENTS 2,811,689 10/1957 Balint 323-60 X 5/1917 OsnOs 336-155 X FOREIGN PATENTS 9/1926 Lucas323-60 4/1940 Minor 321 60 X 5 578,849 6/1963 Germany. 7/1940 Bohm323-60 X 8/1940 Fries 3 60 X LLOYD MCCOLLUM, Pllmary Examiner. 12/1942Fries 323-60 X MILTON O. HIRSHFIELD, Examiner.

7/ 1946 Peterson i 323-60 b 6/1950 Smeltzly 323 6 10 W. E. RAY,Asszstant Examzner.

1. A TRANSFORMER SYSTEM FOR SUPPLYING POWER TO A LOAD FROM A CONSTANTVOLTAGE A.C. INPUT COMPRISING A CORE OF A SINGLE TYPE OF FERROMAGNETICMATERIAL HAVING TWO OUTER LEGS AND A CENTER LEG, SAID LEGS BEING OF SUCHCONFIGURATION AND SO ALIGNED WITH EACH OTHER THAT SUBSTANTIALLY CLOSEDHIGH PERMEABILITY PATHS ARE FORMED BETWEEN THE OUTER LEGS AND THE CENTERLEG, SAID CENTER LEG BEING SEPARATED BY NON-MAGNETIC MATERIAL INTO TWOPARALLEL PORTIONS ALSO PARALLEL TO SAID PORTIONS OF THE OUTER LEGS, SOTHAT THE RELUCTANCE OF THE PATH BETWEEN SAID PARALLEL PORTIONS OF THECENTER LEG IS EXTREMELY HIGH IN COMPARISON WITH THE RELUCTANCE OF SAIDSUBSTANTIALLY CLOSED PATHS; AT LEAST THREE COILS WOUND ON SAID CORE, AFIRST AND A SECOND OF SAID COILS BEING WHOLLY WOUND ON DIFFERENT ONES OFSAID OUTER LEGS AND CONNECTED IN SERIES, THE THIRD COIL BEING WOUND WITHEACH TURN SURROUNDING BOTH OF AND ONLY SAID PARALLEL PORTIONS OF THECENTER LEG; AN INPUT, AN OUTPUT, AND A CONTROL ELECTRICAL CIRCUIT, ALLPASSING CURRENT, THE INPUT CIRCUIT INCLUDING ONE OF (A) SAID THIRD COILAND (B) THE SERIES COMBINATION OF SAID FIRST AND SECOND COILS, THE INPUTCIRCUIT BEING CONNECTED ACROSS SAID CONSTANT VOLTAGE A.C. INPUT, THEOUTPUT CIRCUIT INCLUDING THE OTHER ONE OF (A) SAID THIRD COIL AND (B)THE SERIES COMBINATION OF SAID FIRST AND SECOND COILS, THE OUTPUTCIRCUIT BEING CONNECTED ACROSS SAID LOAD; SAID